Oktober - Senwes Tuisblad

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10 ••• aGri TEcHNiqUE reproduction in domesticated and wild animals is a complex process and is affected by a variety of environmental and PHYSIOLOGICAL FACTORS. > text: ProF eDDie webb > this complexity was well summarised by Dr Hansel from the Cornell University: “It is not a wonder that reproduction sometimes fails, but rather a miracle that so many pregnancies terminate successfully.” Man knows several thousand mammalian species, but reproduction has only been studied in approximately 25 species. This complex process is characterised by a number of peculiar reproductive phenomena, mainly to ensure the perpetuation of the species. These phenomena (summarised in Figure 1) ensure that CharaCteriseD by PeCuliar reProDuCtiVe Phenomena restriCteD sexual season absenCe oF estrus DissoCiation oF oVulation PresenCe oF menstruation non-sPontaneous oVulation sPontaneous multiPle oVulation with limiteD imPlantation oVulation During PregnanCy DelayeD imPlantation Figure 1: reproductive phenomena of domesticated and wild animals. Practical methods reproduction www.senwes.co.za • <strong>Oktober</strong> 2010
to manage the of beef cattle the progeny is born when the environment is conducive to maximum survival. For example, the mother can sufficiently nourish and care for the progeny, adequate nutrition is available to the mother and progeny and environmental stressors are minimal. Livestock like cattle, sheep, goats and pigs were domesticated over many centuries and in different countries to meet the needs of humans for food (meat, milk and cheese), fibre and clothing – to mention just a few. These species vary in terms of sexual season, sexual cycle, reproduction rate, gestation period, type of placentation, recovery after parturition (involution), litter size, lactation period and susceptibility to reproductive diseases. Seasonal effects are generally not as evident in tropical as opposed to temperate areas and also less evident in domesticated livestock compared to their wild counterparts. Fertility The fertility of animals is directly dependant on the activity of the gonads (reproductive organs) and accessory sex glands. In turn, the activity of the gonads is influenced by hereditary or genetic factors as well as a number of environmental factors like the ambient temperature, humidity, photoperiod (day length) and nutrition. In addition, hormonal balance is a prerequisite for optimum reproduction and this is regulated by delicate interactions and feedback mechanisms aGri TEcHNiqUE ••• between the central nervous system (CNS), pituitary gland and gonads. The hypothalamus is regarded as the “master gland”, the pituitary gland as the “conductor of the endocrine orchestra”, while the environmental stimuli constitute the “music that it plays”. The endocrine messages are mediated through the gonadotrophins or sex hormones. Alterations in this delicate system may have farreaching consequences, often on apparently unrelated organs or tissues (Bonsma, 1980). At conception, the complete genetic potential of the animal is laid down and the potential boundaries within which it can function and produce during its entire lifetime is determined. It is accepted that the phenotype of the animal is determined by the complete interaction between the genotype and the external environment, but the latter also affects the physiological functioning, adaptation, reproduction and potential performance of animals. The heritability of fertility is estimated at approximately 10%, which means that only 10% of the variability in fertility is under genetic control, while the remaining 90% of the variation is determined by the external environment. Heritability estimates for the components of fertility (like scrotal circumference and semen characteristics in males) appear to be higher. continued on page 13 11
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